In wireless communication networks congestion control is an issue that needs to be addressed in order to have a network that operates smoothly thereby providing good quality of service (QoS) to its users. By “congestion” in communication networks is usually understood a situation, where the network is unable to deliver the offered load due to the insufficient capacity or overloaded traffic. The results of congestion are the degradation of QoS for the services, i.e. lower throughput, higher end-to-end packet delay, or even discard of packets in some severe cases.
Communication networks are known, which can provide two different types of services in terms of safety. First class of services includes only services related to safety or security, whereas the other service type provides other types of services, i.e. services that are not related to safety. An example of this kind of system is a wireless inter-vehicle communications (IVC) system, which is intended for improving the safety and comfort of driving. Generally, the services provided by IVC systems fall into two categories, namely safety services and non-safety services.
In IVC systems safety services provide the driver with information about critical situations in advance. Typical applications of this category include traffic signal violation warning, emergency electronic brake lights, pre-crash warning, cooperative danger warning dissemination, lane change warning, etc. Since closely related with human safety in high speed vehicular environments, these kinds of applications are characterised by the strict requirements on reliability and latency that is usually less than 100 milliseconds. On the other hand, intended for enhancing the driving comfort and improving the efficiency of transportation system, the non-safety services are usually assigned with lower priorities compared to the safety services. This category of applications, including traffic message dissemination, toll collection, electronic map download, multimedia applications and so on, usually require more bandwidth.
Owing to the ability of offering high-rate data transmission with low latency in a local range of several hundreds meters up to one kilometer, the Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless local area network (WLAN) technology is widely accepted as the enabling technology of medium access control (MAC) and physical layer (PHY) for the next generation IVC system. The IEEE WLAN MAC is based on carrier sense multiple access with collision avoidance (CSMA/CA), where each station follows a listen before talk and random backoff scheme to avoid collisions, as the distributed coordination function (DCF) specified in IEEE 802.11 standard. The priority of safety services is granted by assigning a relatively shorter channel sensing time and smaller random backoff slot number generating window, as the enhanced distributed channel access (EDCA) function specified in the IEEE 802.11e standard. However, in systems where the channel resource is shared by services with a priority of the first order, such as safety applications, and services with a priority of the second order, such as non-safety applications, e.g. IEEE 802.11p, the QoS of the safety applications may be compromised due to the channel congestion caused by the non-safety applications. In these systems, where the channel resource is shared by safety and non-safety services, non-safety applications may exhaust the channel resource, even if a higher priority has been assigned to the safety applications. Thus, it is desirable to avoid congestion in order to prevent the QoS degradation of services with a priority of the first order.
Thus, there is a need for an improved congestion control method.